1. Field of the Invention
The present invention relates generally to group-III Nitride semiconductor alloys and system and methods for manufacturing same.
2. Description of the Related Art
Since the understanding of the physics of the blue light emitting diode, (“LED”), the nitride materials system has been meticulously investigated for its applications in optoelectronic devices. Group-III Nitride semiconductor alloys, such as Indium-Gallium-Nitride (“InGaN”) alloys, are researched because the direct band gap of InGaN can be tuned to allow emission or absorption from the entire visible spectrum by adjusting the indium content. Specifically, the tunable direct band gap of InGaN between 0.7 eV (InN) and 3.4 eV (GaN) makes this alloy particularly intriguing for both light-emitting and photovoltaic materials. High breakdown fields common among all group-III nitrides have been exploited for currently commercialized high-power electronics. These strengths could be further utilized in space applications due to the substantial radiation hardness of InGaN.
Currently, there exists no native substrate for InGaN growth. Inherently, InN has different lattice spacings than GaN, and thus no conventional substrate can be completely lattice matched to all alloys of InGaN. InGaN LED structures are typically grown by metalorganic chemical vapor deposition (“MOCVD”) on sapphire or SiC substrates that have substantial lattice mismatches to all nitride alloys. The dislocations created by this mismatch are partially mitigated by preparing GaN templates, which reduce or eliminate many defects by growing a thick, GaN buffer. This technique allows for the growth of thin, low indium content InGaN quantum wells to be grown completely strained and without relaxation on the thick GaN layers. Although the use of a GaN template only reduces dislocations in GaN for standard, e.g. .about.4 .mu.m, buffer layer thicknesses to 10.sup.8-10.sup.9 cm.sup.3, GaN-based LEDs still exhibit very high efficiencies due to localization of carriers in quantum wells. However, higher indium content quantum wells to push light emission further into the green will strain relax, producing more non-radiative recombination centers. This technique provides a solution for violet and blue LEDs, but both lower wavelength emission and more complex structures such as solar cells and laser diodes require a different approach for the substrate of InGaN. Although there are commonly used methods to circumvent, or at least ameliorate, lattice matching issues to utilize InGaN in violet/blue LEDs, there still exist some core material properties that make high-quality, high indium content InGaN difficult to grow using conventional processes.
InGaN alloys tend to have some form of phase separation through three main degradation processes: thermal decomposition, spinodal decomposition, and indium surface segregation. Although thermal decomposition is a factor in a typical growth process for group-III nitride materials, the effects can be significant for InGaN growth depending on growth conditions. The bonds between indium and nitrogen are weaker than that of gallium and nitrogen, so less energy in the form of heat is required for the bonds to break. In other words, in processes that require a certain temperature to form GaN, the temperature can be detrimental to maintaining the InN bonds formed because the higher temperature increases the probability that the InN bonds will separate. Because of the inherent temperature limitations in growing InGaN alloys, this property introduces inherent process limitations. For example, MOCVD growth uses elevated temperatures to crack ammonia into nitrogen. The relatively high-temperature process of MOCVD during InGaN growth has been shown to cause the dissociation of indium from the InGaN alloy, leaving excess indium metal on the surface. But, if the temperature were reduced in an effort to reduce the dissociation of indium, not only would less reactive nitrogen be available, causing less than ideal growth nitrogen content in the reaction chamber, but the material quality would also suffer because, as less energy is supplied, the energy available to the reactant atoms to find optimal locations in the crystal lattice would be reduced.
Another factor in the growth of InGaN alloys is spinodal decomposition. Spinodal decomposition occurs when phases of materials undesirably separate due to low energy barriers. This is especially problematic for InGaN alloys because InGaN alloys have a region between the InN and GaN phases that, at certain temperatures and indium compositions, has no energy barrier for phase separation. This can cause both minor indium compositional fluctuations as well as two completely separated, more energetically favorable, phases. Complex structures, such as solar cells and laser diodes, require both smooth band structures and very low dislocation densities, both inconsistent with alloys affected by spinodal decomposition. Spinodal decomposition is typically considered a kinetically limited effect, occurring primarily on the surface as bulk diffusion in nitrides is very low.
Indium surface segregation is perhaps the most disruptive of the natural limitations of the nitride materials system. Indium surface segregation can be described as the tendency of indium atoms to preferentially migrate vertically along the growth front and laterally across the film during the growth of InGaN alloys. Vertical indium surface segregation is the competition between indium and gallium on the film surface, where it is more energetically favorable for gallium atoms to occupy surface sites. The vertical surface segregation of indium has been shown to result in a decrease in indium incorporation and blurring of InGaN/GaN interfaces in quantum well growth. A result of the vertical surface segregation, vertical inhomogeneities in quantum wells, negatively impact light emission due to wider well width, causing less than ideal electron-hole wave function overlap.
Lateral segregation of indium is caused by the higher stability of In—In bonds than In—Ga bonds, encouraging indium to laterally aggregate into islands (or pools) if given enough time and mobility on the surface. This migration of indium absorbed atoms (“adatom”s) into clusters on the growing surface has been shown to produce indium-rich nanostructures within the film. Unlike vertical indium surface segregation, lateral surface segregation has been shown to increase light emission from indium-bearing materials. The quantum-dot-like nanostructures created from this form of indium surface segregation have been shown to enhance spatial carrier localization, producing radiative recombination centers. Although perhaps beneficial for LEDs and laser diodes, lateral surface segregation of indium is detrimental for nitride solar cells, where recombination of carriers is preferentially minimized.
To ameliorate limitations caused by segregation, some conventional processes use indium-rich conditions. Indium-rich conditions have been used in the growth of GaN, and InN, where indium adlayers have proven to be useful as a surfactant without affecting material compositions. However, for the case of InGaN, excess indium accumulation has a significant effect on indium incorporation. At the temperatures generally used for InGaN growth, metallic indium has been shown to be difficult to remove from the growth surface in situ without causing heat-induced phase separation, i.e. spinodal decomposition, of the InGaN alloy. Suppression of indium surface segregation has been conventionally achieved by the use of nitrogen-rich growth conditions. But the use of nitrogen-rich growth conditions can prohibit a metal adlayer from accumulating on the growing surface, resulting in a reduction in material quality and surface smoothness. To compensate for issues presented in metal-rich conditions, stoichiometric growth conditions can be used to increasing crystal quality and surface smoothness. But stoichiometric growth conditions are often difficult to achieve uniformly across large wafers at temperatures used for InGaN growth where an intermediate regime does not exist.
In addition to MOCVD, there are other conventional methods for growing InGaN alloys. Molecular beam epitaxy (“MBE”) is a method of growing layers of various materials in a high vacuum or ultra-high vacuum environment. U.S. Pat. No. 7,115,167 discloses the growth of InGaN alloys in a high vacuum growth chamber at temperatures ranging from 650.degree. C. to 850.degree. C. using gaseous ammonia as the source of reactive nitrogen. The nitrogen in the ammonia is separated from the hydrogen atoms through a process called “cracking”. Energy is supplied to the ammonia molecule in the form of heat as the ammonia lands on the growth surface of the substrate, causing the ammonia molecule to break apart. The nitrogen is reactive with at least a portion forming a nitride film.
Typically, the hydrogen portion of the ammonia quickly evaporates from the growth surface and does not appreciably affect the growth. High temperatures are used in the MBE process to produce enough reactive nitrogen for appreciable growth. But, because of the relatively high temperatures used in the MBE process, the alloy formed typically will experience conditions favorable to thermal and spinodal decomposition, reducing the quality of the alloy or preventing the desired indium content. U.S. Pat. No. 7,608,532 discloses a different method using a variation of chemical vapor deposition (“CVD”). The reactive nitrogen source also is ammonia with typical process temperature ranges exceeding those commonly found in MBE processes, also introducing conditions favorable to at least thermal and spinodal decomposition.
It can be seen that there remains a need for an InGaN alloy growth process that achieves the goal of high indium content alloys while accounting for the effects of thermal decomposition, spinodal decomposition, and indium surface segregation. There also remains a need for relatively fast indium growth to alleviate the issues of spinodal decomposition, which is considered a kinetically limited effect. It is to the provision of such a process that the present invention is primarily directed.